Experiments in this grant application are designed to build upon classic studies documenting powerful effects of early handling and maternal separation on adult stress-induced hormone secretion. There is a particularly large literature documenting how adverse neonatal experiences have major, long-term effects on the physiology and behavior of many species, including depression in humans. However, little is known about the impact of neonatal rearing environment on future, adult responses to stressful situations in the mouse. Understanding these environmental effects on mouse brain development would allow us to take advantage of powerful genetic tools available in the mouse to determine the interaction between genetic and epigenetic factors governing brain development. Because maternal separation procedures vary, research in this proposal will optimize the parameters of an early life stress model that exerts permanent long-term effects on the hypothalamic-pituitary-adrenal (HPA) axis in adult male C57BL/6 mice. These experiments will characterize the time of day that neonatal early handling has the maximal response on adult behavior,hormone secretion, and mRNA expression, and whether the presence of littermates can buffer the effect of neonatal maternal separation on adult behavior, hormone secretion and mRNA expression in C57BL/6 mice. It is important to note that the early life stress model that alters stress hormone secretion in adult rats may not be the ideal model system for eliciting changes in adult mice. Therefore, the experiments outlined in this proposal are essential before conducting work exploring the influence of environment on genetic mutants on the C57BL/6 strain of mice. At the conclusion of the experiments in this R03 grant, we will understand the importance of critical variables during differential rearing, such as time of day and isolation from siblings, for altering maternal behavior, adult behavior, stress-induced adrenocorticotropin hormone and corticosterone secretion, and mRNA expression of regulators of the HPA axis in the C57BL/6 mouse. Finally, the basic research outlined in this proposal has clinical biomedical relevance and fulfills an aim of NIMH to decrease the burden of mental illness. Increasing evidence suggests that early adverse experience contributes to the etiology of depression, and this mouse differential rearing model could therefore be helpful in understanding how early life stressors alters neural circuitry that then influence an individual's susceptibility to depression. [unreadable] [unreadable] [unreadable]